Well known in the prior art are multiple speed bicycles (e.g., a "ten-speed") which generally comprise a bicycle frame consisting of a top tube, down tube, seat tube, a pair of seat stays, and a pair of chain stays. The top, down and seat tubes generally define a front triangle of the bicycle frame, with the seat tube, seat stays and chain stays collectively defining a rear triangle of the bicycle frame. The back ends of corresponding pairs of the seat and chain stays are attached to respective ones of an opposed pair of drop-outs adapted to accommodate the rear wheel axle of the rear wheel. In prior art multiple speed bicycles, the bottom ends of the down and seat tubes and the front ends of the chain stays are typically attached to a sleeve which rotatably accommodates a bottom bracket axle. Attached to the bottom bracket axle is a pair of pedal cranks, each of which includes a pedal roatatably connected thereto.
In addition to the pedal cranks, attached to the bottom bracket axle are multiple chain wheels of differing diameters. Similarly, attached to the rear wheel axle is a sprocket cluster consisting of multiple sprockets of differing diameters. One of the chain wheels is mechanically coupled to one of the sprockets of the sprocket cluster via a drive chain. In the prior art multiple speed bicycles, the drive chain may be selectively shifted onto any one of the chain wheels via a front derailleur which is typically attached to the seat tube. Additionally, the drive chain may be selectively shifted to any one of the sprockets of the sprocket cluster via a rear derailleur which is typically attached to one of the dropouts.
Though prior art multiple speed bicycles have been in existence for many years, they posses certain deficiencies which detract from their overall utility. More particularly, the front and rear derailleurs of the bicycle are of complex construction and typically include multiple parts or components. As such, the derailleurs are extremely susceptible to mechanical failure or breakage, particularly when the bicycle is used in off-road conditions as generally occurs with multiple speed mountain bikes. Additionally, because of their relatively complex construction, the front and rear derailleurs are also susceptible to being displaced from their normal orientations and falling out of proper adjustment when exposed to debris such as dirt or rocks, or when subjected to a sudden impact force as typically occurs when the rider falls. Any such displacement of the front or rear derailleurs usually causes the drive chain to be completely disengaged from the chain wheels or the sprockets of the sprocket cluster when shifting between the chain wheels or the sprockets is attempted by the rider. The re-adjustment of the front and rear derailleurs or the repair thereof in the event of breakage is usually both expensive and time consuming. Additionally, because of the relatively large number of sprockets typically included in the sprocket cluster on the rear wheel axle, the rear wheel cannot be provided with a wide "stance", which would assist in the handling of the bicycle.
The present invention addresses the deficiencies of prior art multiple speed bicycles by providing a direct drive bicycle which includes a variable rate transmission unit. The inclusion of the transmission unit within the bicycle of the present invention provides multiple speed capability, while eliminating the multiple chain wheels, sprocket cluster, and front and rear derailleurs of conventional multiple speed bicycles. In this respect, the direct drive bicycle of the present invention includes only a single chain wheel and a single sprocket attached to the rear wheel thereof. The transmission unit is itself mounted within the main frame or front triangle of the bicycle which protects the same from exposure to and thus potential damage from debris such as dirt and rocks, and further shields and protects the transmission unit in the event the rider falls. The direct drive bicycle of the present invention is further provided with a shock absorbing rear suspension system which enhances its performance in off-road conditions.